Cationic electrodeposition painting makes it possible to paint details of a painting target even if the painting target has a complicated shape, and to paint a plurality of painting targets automatically and continuously. Accordingly, it has come in wide use for executing an undercoat painting method that particularly enables coating on a large painting target having a complicated shape, such as an automobile body.
Deposition of a coating film in a process of the cationic electrodeposition painting is caused by an electrochemical reaction, in which the coating film is deposited on a surface of the painting target by application of a voltage. The deposited coating film has insulation properties. Accordingly, in a painting process, electrical resistance of the coating film is increased as the deposition of the coating film progresses and the deposited film grows in thickness. As a result, the amount of deposition of the paint in a region where the coating film has been already deposited is lowered, and instead, on a region where a coating film has not yet been deposited, deposition of the coating film starts. As described above, a solid content of the paint is sequentially deposited onto the painting target, whereby the painting is completed. In this specification, such properties where the coating film is sequentially formed on undeposited regions of the painting target are hereinafter referred to as “throwing power”.
In Patent Literature 1, an electrodeposition coating film forming method is described, which is capable of enhancing the throwing power without being accompanied with deterioration of an exterior appearance of the coating film owing to generation of a gas pinhole, and the like.
However, the electrodeposition coating film forming method has properties that, in a case where the painting target has a narrow gap in terms of a structure thereof, as a target portion is being located in a deeper side from an opening portion of the gap, deposition of a coating film becomes more difficult. In particular, in a case where the structure of the gap has a hermetically sealed bag portion with a cylindrical or rectangular parallelepiped shape or the like though the opening portion, and particularly in the deeper side from the opening portion thereof as the target portion is being located more in an inside of the gap, deposition of a coating film becomes more difficult. For example, in a case where the painting target is a structure formed by connecting a plurality of steel plates together, and each of connected portions of the plurality of steel plates has a gap between one steel plate and another steel plate stacked on and connected to the former, formation of the coating film in an inside of the gap is difficult, and the painting becomes prone to be imperfect. An inside of such a narrow gap formed in the painting target is generally referred to as “a clearance portion”.
In general, when a gap between one steel plate and another steel plate becomes 300 μm or less, quality of deposition of electrodeposition coating films into the gap is obviously deteriorated. Moreover, in a case where a gap like that described above is 100 μm or less, quality of deposition of electrodeposition coating films is further deteriorated, and if a distance of a target portion from an opening portion of the gap is 5 mm or more, then a portion where the coating film is not formed remains on such a depth of the gap.
As described above, in a case where a portion in which the electrodeposition painting is imperfect is left somewhere on a surface of each steel plate, it is likely to generate rust at the portion concerned.